1. Field of the Invention
The present invention relates to a vehicle comprising a steering operation means, a pair of running-driving wheels which are differentially driven when the steering operation means is operated for cornering, and a pair of steerable running wheels interlocking with the steering operation means.
2. Background Art
Conventionally, technology where a pair of hydrostatic transmissions (HSTs) are laterally connected, driving axles project laterally from respective HSTs, running-driving wheels are fixed to the outer ends of both axles, wherein movable swash plates as capacity adjusting members for the hydraulic pumps of the HSTs are individually changed in angle thereby driving the left and right running wheels individually, is well-known, as disclosed in, for example, the U.S. Pat. No. 4,782,650.
In such construction, running speeds of the left and right HSTs, when the vehicle is driven straight forward, are equalized, and, when turned, are different.
The above-said vehicle, however, could not travel straight-forward unless the output rotations of left and right HSTs completely coincided with each other, adjustments in shipment took much time, and parts and assembly errors had to be diminished so as to improve accuracy. Also, when there was a difference between the capacities of hydraulic pumps and motors, left or right turning feeling of the vehicle was different, resulting in that the vehicle was very hard to steer.
Thus, for overcoming the above problems, a vehicle including a steering operation means; a pair of running-driving axles; a pair of running wheels drivingly connected with the pair of running-driving axles; a first differential unit interposed between the pair of running-driving axles; a first hydrostatic transmission for transmitting a driving force to the first differential unit; a pair of steering output shafts; a second differential unit interposed between the pair of steering output shafts; a second hydrostatic transmission for transmitting a driving force to the second differential unit; a first drive train interposed between one of the steering output shafts and one of the running-driving axles, and a second drive train interposed between the other steering output shaft and the other running-driving axle for transmitting the rotating effort to the other running-driving axle in the opposite direction to the first drive train, wherein the second hydrostatic transmission operationally interlocks with the steering operation means so that the output speed and direction of the second hydraulic transmission is changed by manipulation of the steering operation means, has come to be invented. The vehicle does not require such labor as above mentioned for precise coincidence between the capacities of the first and second hydrostatic pumps and motors. Also, when both the hydrostatic pumps and motors are arranged in a longitudinal line, the vehicle becomes laterally compact.
It is still desirable to improve the running efficiency of such a vehicle when it drives and turns on a rough road or a soft ground. If the vehicle is provided with a caster in addition to the pair of running wheels, it can turn in a small circle, however, cornering on a rough road or a slope becomes unstable. It is good for steady cornering that the vehicle has steerable running wheels which can be turned laterally by operating the steering operation means. However, the steerable running wheels restrict the reduction of cornering circle. It is impossible for the steerable running wheels to have such a small cornering circle that can be ensured by casters. Especially, it is further hard for the vehicle to turn on a determined radius circle steadily when the vehicle has a plurality of running wheels arranged in a longitudinal direction so as to be made larger in whole length.